Valleys

S
everal deep rifts traverse the eastern and southern parts of Zambia, forming the southern end of the great East African rift system. These rifts, or troughs as geologists prefer to call them, vary in depth. The two deepest are the valleys of Lakes Tanganyika and Malawi, which, with Lake Baikal, are the world’s deepest lakes.

The valleys of the middle Zambezi and the Luangwa and its tributaries, the Lukusashi and Lunsemfwa, are all approximately 300m in depth. The Kafue Flats form yet another valley trough, although the altitude, about 975m, is only slightly less than the surrounding plateaux, and there are only minor escarpments.

All of these valleys have been formed by down-faulting. The rocks of the valley floors date from the Karroo period. Fossil bones of mammal-like reptiles (Therapsids), which preceded the dinosaurs, have been found in a few places.

Soils derived from the Karroo sandstones generally have a higher mineral content than those derived from the basement complex of the plateaux, and the contrast in the vegetation is sharp. The Luapula Valley is not part of the rift valley system and belongs to the basement complex. Its natural vegetation is not miombo, and although it resembles the vegetation of the other valleys in structure, the species are quite different.

Valley vegetation consists of complex mosaics. It is affected by the drainage pattern and soils, and also by large herbivores which are concentrated in these nutrient-rich areas. Deciduous thickets commonly occupy the well-drained sites. The banks of rivers and lagoons have riverine fringing forest. The slopes between the thickets and the riverine forest are frequently covered with mopane woodland. Grassy plains occur on cracking clay soils. Lagoons, which are frequently formed as ox-bow lakes, have a rich variety of aquatic vegetation. At the lower end of the mopane slopes there are frequently large termite mounds covered with forest species. These mounds are often partly or completely encircled by pans, which hold water for several months into the dry season. These pans begin as wallows and are extended as more mud is carried out on animals using them.

Mopane and Mopane Woodland

The mopane tree (Colophospermum mopane) is a very distinctive species familiar to anyone who has visited the Luangwa or Zambezi Valleys, where it forms extensive pure stands on the valley floor and lower escarpment slopes. Most people associate mopane with the hot dry valleys, but it also occurs quite extensively on the southern plateau. Its distribution in Zambia is strongly correlated with Karroo sandstone, dating from the Triassic period, which occurs in a number of down-faulted troughs in the much older surrounding basement complex.

In the drier parts of its distribution range mopane may dominate most soil types, but on the plateau it is confined to particular soil types, which are alkaline and contain high concentrations of sodium salts. The clays in these soils swell on absorbing water, and rapidly become completely impervious.

These conditions are unfavourable to the growth of most trees, and the few species that do tolerate them must be adapted to take up water rapidly for the short time it is available. In fact mopane develops a superficial root system which is able to suppress perennial grasses, and it is not uncommon to observe isolated trees in a circle of taller perennial grass, with only sparse annual grasses and herbs under the trees. This suppression of perennial grasses actually promotes surface runoff and soil erosion, and gulleying is a common feature of mopane woodland areas.

The soil characteristics of mopane woodland are in complete contrast to those of miombo woodland, which conserves both soil and water. Yet mopane can grow on deep, well-drained soils, and many of the finest specimens are on such soils. In these circumstances it develops a deep taproot like its woodland associates. Yet it is evident from its distribution that it cannot compete with Brachystegia species and the other miombo dominants.

Besides miombo and mopane there are other woodland types, mostly of minor occurrence. They are generally more open in structure than miombo, and lack the characteristic miombo dominants. They occur in situations which are either too dry for miombo, or become too wet during the rains, or suffer from fires too severe for the miombo species to tolerate.

Animal-modified Vegetation

The unnaturally high population densities of herbivores in the more popular National Parks have greatly modified the natural vegetation. The constant trampling and browsing of animals effectively prevents any but the best protected species from getting started. Most trees become established as seedlings in dense thickets, which are not attractive to large animals. In fact we often associate elephant, black rhino and buffalo with dense thickets, but this is an artificial situation brought about by hunting with firearms, which has forced these animals to take shelter in the dense forests and thickets. When left in peace these animals choose more open habitats and avoid the thickets.

Trees such as the baobab (Adansonia digitata), Cordyla africana, and several other fruit trees become established in thickets, but once these trees mature and their ripe fruits fall to the ground, elephants and other heavy herbivores attracted to the fruits open up the thicket and transform it into a parkland, which is what we see in many of the best game viewing areas in the Luangwa and Lower Zambezi National Parks. Since there is no way that these trees can replace themselves their habitats are not sustainable.

Other trees are able to regenerate without the protection of thickets. These include Acacia tortilis, which forms its own spiny thickets, and the two fanpalms, Borassus aethopum and Hyphaene petersiana, which have very coarse foliage that resist browsing.